1. Field of the Invention
The present invention relates to a pulse wave sensor for detecting pressure pulse wave from an artery of a living subject.
2. Related Art Statement
There is known a pulse wave sensor having a press surface adapted to be pressed on a body surface of a subject, a protruding portion protruding from the press surface, and pressure sensing means provided in a top surface of the protruding portion. An example of this sensor is disclosed in U.S. Pat. No. 4,901,733 published on Feb. 20, 1990, the Assignee of which is the Assignee of the present application. When the protruding portion is pressed on the body surface over a superficial artery such as radial artery or dorsal artery of foot, the pressure sensing means detects pressure pulse wave produced in the artery in synchronism with heartbeat of the subject. Although in many cases bone and tendon occur near a superficial artery from which pressure pulse wave is to be detected, the pressure sensing means provided in the top surface of the protruding portion appropriately is pressed against the artery without being interfered by the tendon or bone.
In order to measure pressure in an artery (i.e, blood pressure) as accurately as possible by detecting pressure pulse wave from the artery, it may be required that the pulse wave sensor be pressed on the body surface with an optimum pressing force which causes the wall of the underlying artery to partially be flattened. If, to this end, the height of the protruding portion as measured from the press surface is selected at a small value to suitably press an artery shallow under body surface, then this arrangement is not suitable for a deep artery, because the sensor must be pressed with an excessively great force for overcoming resistance or interference between the other portion of the press surface than the protruding portion, and the bone and/or tendon near the artery. This results in causing the subject to feel discomfort or even pain. In addition, the sensor may fail to establish an optimum pressing force which partially flattens the artery. On the other hand, if the height of the protruding portion is selected at a great value to suitably press a deep artery, then the arrangement is not suitable for a shallow artery, because the sensor must be pressed with a small force as an optimum pressing force. In this case, the other portion of the press surface than the protruding portion does not contact the body surface. Thus, the sensor takes an unstable posture or position, and therefore the sensor does not detect pressure pulse wave with stability. Since the depth of a superficial artery such as radial artery or pedal dorsal artery finds differences among individual subjects, a predetermined height of the protruding portion of the pulse wave sensor may cause some subjects to feel discomfort, may fail for some subjects to establish an optimum pressing force, and may not stably detect pressure pulse wave from other subjects.
In the above-identified background, the Assignee of the present application proposed, in Japanese Utility Model Application No. 2-46420 filed Apr. 27, 1990, to provide a soft deformable member, such as a compressed deformable rubber, around a protruding portion on a press surface of a pulse wave sensor. The soft deformable member has a height generally equal to, or greater than, that of the protruding portion. When the protruding portion presses a deep artery via body surface, the soft deformable member is deformed by compression thereof between the press surface and the body surface, so that the protruding portion protrudes from the level of the top of the deformed member by an amount or distance corresponding to the depth of the artery. This arrangement appears to eliminate the above-indicated problems with the first pulse wave sensor.
However, even the second pulse wave sensor is not satisfactory, because the amount of compressed deformation of the soft deformable member (i.e., spring characteristic) is not linear with respect to load applied thereto. Therefore, for a deep artery, the soft deformable member does not easily produce a sufficient deformation, that is, does not easily cause the protruding portion to project therefrom by a sufficient amount corresponding to the depth of the artery. Therefore, an excessively great pressing force is applied to the sensor to establish an optimum pressing force which partially flattens the artery. Consequently, this sensor may cause the subject to feel discomfort due to the excessively great pressing force applied thereby.